System and method of small field of view x-ray imaging

ABSTRACT

A system and method of X-ray imaging includes an X-ray emitter that projects X-rays. An X-ray receiver receives X-rays from the X-ray emitter to produce a plurality of projection images. A filter with at least one filter leaf absorbs at least a portion of the X-rays from the X-ray emitter to define a limited field of view within a full field of view, wherein the X-rays are attenuated in at least one attenuated portion of the full field of view. A processor reconstructs a three dimensional image based upon the projection images of the full field of view. The limited field view is located within the reconstructed three dimensional image. At least one corrective parameter is determined from the reconstructed three dimensional image. A three dimensional image is reconstructed based upon the limited field of view and the at least one corrective parameter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/611,865, filed Feb. 2, 2015, which application was published on Aug.4, 2016, as U.S. Publication No. US20160220201 the contents of which areincorporated herein by reference in their entirety.

FIELD

The present disclosure relates to X-ray imaging systems and methods.

BACKGROUND

U.S. Pat. No. 8,155,415 discloses an apparatus and method for expandingthe field of view of a truncated computed tomography (CT) scan. Aniterative calculation is performed on the original CT image to producean estimate of the image. The calculated estimate of the reconstructedimage includes the original image center and an estimate of thetruncated portion outside the image center. The calculation uses animage mask with the image center as one boundary.

U.S. Pat. No. 8,107,708 discloses a method for correcting truncationartifacts in a reconstruction method for computed tomography recordings.The projection images are recorded by an X-ray image detector beingextended by determining the attenuation of the radiation outside theprojection image for pixels. Non-horizontal filter lines are extended bytransaxial and axial artificial extension of the X-ray image detectorfor the purposes of truncation correction.

U.S. Pat. No. 7,756,315 discloses a method for expanding a field-of-viewof a volumetric computed tomography scan. The method includesidentifying truncated views having projection truncation andnon-truncated views without projection truncation based on an averagevalue of one or more edge channels. An estimated missing projection iscalculated for each of the truncated views based on at least oneneighboring non-truncated view. A projection profile is calculated foreach of the truncated views based on the estimated missing projection,and the projection profile provides at least one of attenuation data andprojection data for an area outside a field-of-view.

U.S. Pat. No. 5,278,887 discloses an apparatus and method for reducingthe dosage of X-rays during a fluoroscopic procedure. A filter member isused to selectively attenuate the X-ray radiation striking a patient'sbody. The filter member allows unattenuated X-rays to image an area ofinterest selected by a physician, thus producing a high intensity, lownoise image. Areas surrounding the area of interest are imaged withattenuated radiation producing a less intense, more noisy image.

U.S. patent application Ser. No. 12/990,285 discloses a method of usingnon-attenuation corrected PET emission images to compensate forincomplete anatomic images. A segmented contour of anon-attenuation-corrected (NAC) PET image is used to identify a contourof the truncated region. An appropriate tissue type is used to fill intruncated regions of a truncated CT or MR image or the attenuation map.The corrected attenuation map is then used to generate anattenuation-corrected PET image of the patient or a region of interest.

U.S. patent application Ser. No. 13/113,714, which is herebyincorporated by reference in its entirety, discloses X-ray imagingsystems and methods that utilize an imaging apparatus that includes anemitter emitting X-rays through an object and a receiver receiving theX-rays. A control circuit controls the emitter and processes the X-raysreceived by the receiver to generate X-ray images of the object. Thecontrol circuit controls a display to display an initial view of theobject. The display of the initial view of the object is modifiable by auser. The imaging apparatus is controlled to generate an X-ray positionimage of the object based upon the user modification of the display ofthe initial view. The display is controlled to display a positioningimage. The display of the positioning image is modifiable by a user andthe imaging apparatus is controlled to generate an X-ray image of theobject based upon the user modification of the display of thepositioning image.

BRIEF DISCLOSURE

An exemplary embodiment of an X-ray imaging system includes an X-rayemitter that projects X-rays through an object which at least partiallyabsorbs the X-rays. An X-ray receiver is configured to receiveunabsorbed X-rays from the X-ray emitter and produces projection imagesof the object from the received unabsorbed X-rays. A filter is disposedbetween the X-ray emitter and the X-ray receiver. The filter includes atleast one filter leaf that absorbs at least a portion of the X-rays fromthe X-ray emitter. The at least one filter leaf is adjustable to adjustan amount of X-ray intensity applied to the object in at least a fullfield of view and a limited field of view. A processor is connected tothe X-ray receiver. The processor executes computer readable code storedupon a computer readable medium and upon execution of the computerreadable code processes the projection images from the X-ray receiver.The processor reconstructs a three dimensional image based uponprojection images of the full field of view. The processor locates thelimited field of view within the reconstructed three dimensional imagebased upon the full field of view. The processor identifies at least onecorrective parameter from the reconstructed three dimensional image. Theprocessor reconstructs a three dimensional image based upon the limitedfield of view with the at least one corrective parameter.

A method of X-ray imaging includes projecting X-rays from an X-rayemitter. The X-rays are filtered with at least one filter leaf thatabsorbs at least a portion of the projected X-rays to define a limitedfield of view within a full field of view. The X-ray intensity isattenuated in at least one attenuated portion of the full field of view.The X-rays are received at an X-ray receiver to acquire a plurality ofprojection images from the limited field of view and the full field ofview. A processor reconstructs a three dimensional image based upon theprojected images from the full field of view. The processor locates thelimited field of view within the reconstructed three dimensional image.The processor defines at least one corrective parameter from the locatedlimited field of view. The processor reconstructs a three dimensionalimage based upon the projected images from the limited field of view andthe at least one corrective parameter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an exemplary X-ray imaging apparatus.

FIG. 2 is a schematic representation of an exemplary embodiment of anX-ray imaging system.

FIG. 3 is a flow chart that depicts an exemplary embodiment of a methodof X-ray imaging.

DETAILED DISCLOSURE

In the present description, certain terms have been used for brevity,clearness and understanding. No unnecessary limitations are to beimplied therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes only and are intended to bebroadly construed. The different systems and methods described hereinmay be used alone or in combination with other systems and methods.Various equivalents, alternatives and modifications are possible withinthe scope of the appended claims. Each limitation in the appended claimsis intended to invoke interpretation under 35 U.S.C. § 112(f), only ifthe terms “means for” or “step for” are explicitly recited in therespective limitation.

FIG. 1 depicts an exemplary X-ray imaging apparatus for acquiring X-rayimages of an object, exemplarily a dental or medical patient. In theparticular example shown, the imaging apparatus 10 is configured for 3-Dimaging of the dentomaxillofacial complex of the human skull; however,other configurations of apparatuses for imaging of other portions of apatient's anatomy can instead be employed with the concepts of thepresent disclosure. The X-ray imaging apparatus 10 can optionally beconfigured to conduct different types of imaging procedures, including,but not limited to panoramic imaging (standard, pediatric, orthozone,wide arch, orthogonal and/or the like), cephalometric imaging (cephalopediatric lateral projection, cephalo lateral projection, cephalopostero-anterior, and/or the like), and/or 3-D imaging. FIG. 1 depictsjust one example of an X-ray imaging apparatus for use with the conceptsin the present disclosure. Other examples of X-ray imaging apparatus canbe instead be employed, including, but not limited to computedtomography (CT) and fluoroscopic imaging. Techniques and apparatusdisclosed herein may also be used in connection with other forms ofmedical imaging and medical imaging modalities, and it is to berecognized that dental imaging is only an exemplary application.

The imaging apparatus 10 includes a housing 12 that is moveablysupported on a support column 14. The housing 12 can be moved up anddown in the vertical direction V via a conventional guide motor (notshown) that is configured to move the housing 12 vertically up and downalong a track 16 extending along the support column 14. The housing 12includes a generally vertically extending guide section 18 disposed onthe support column 14 and a generally horizontally extending supportsection 20 extending generally horizontally from the guide section 18.The support section 20 supports a rotating section 22, which isrotatable in a horizontal plane H with respect to the stationary supportsection 20. The support section 20 and/or rotating section 22 maycontain a conventional guide motor (not shown) configured to rotate therotating section 22. In an alternative embodiment, the imaging apparatus10 can be mounted to a support structure (not depicted) exemplarily awall instead of, or in addition to, being supported by the column 14.

An X-ray emitter housing 24 and an X-ray receiver housing 26 arepositioned on opposite ends of rotating section 22 and extend generallyvertically downward from the rotating section 22. The emitter housing 24contains an X-ray emitter generally located at 28, although notdepicted, and supported in the emitter housing 24. The X-ray emitter ispositioned to emit X-rays from the X-ray emitter through the objectbeing imaged (e.g. the patient) to an X-ray receiver generally locatedat 30, although not depicted, supported in the X-ray receiver housing26. A patient positioning housing 32 extends from the guide section 18and includes a chin support 34 for positioning the head of the patient(not depicted) between the opposed X-ray emitter 28 and the X-rayreceiver 30. A head support 36 extends from the horizontal supportsection 20 through the rotating section 22. The chin support 34 and thehead support 36 may be optional, and positioning of the patient may becarried out in alternative manners. Embodiments of the imaging apparatus10, further include a graphical display 38, for presentation of imagesas disclosed herein. In additional embodiments, the graphical display 38may be a touch-sensitive graphical display that further operates as aninput device to receive user input or control commands for the imagingapparatus 10.

FIG. 2 is a schematic representation of an exemplary embodiment of anX-ray imaging system 40. As briefly described above, the X-ray imagingsystem 40 includes an X-ray emitter 42 and an X-ray receiver 44. TheX-ray receiver 44 is spaced apart from the X-ray emitter 42 toaccommodate an object, exemplarily a patient's head P, between the X-rayemitter 42 and the X-ray receiver 44. A beam 46 of X-rays is projectedfrom the X-ray emitter 42 to the X-ray receiver 44 passing through theobject P disposed there between. In a non-limiting embodiment, the beam46 is a cone beam, although it would be recognized that alternativeembodiments may use other beam shapes, including, but not limited to fanbeam or line beams as may be recognized by one of ordinary skill in theart.

In operation, the X-ray emitter 42 projects the beam 46 in the directionof X-ray receiver 44. The X-rays pass through the patient's head P andthe anatomical structures of the patient's head P absorb varying amountsof the X-rays. After passing through the patient P, the attenuatedX-rays are absorbed at the X-ray receiver 44 that converts the intensitypattern of the received X-rays into a digitized output representative ofthe unabsorbed X-rays at the X-ray receiver 44. The X-ray receiver 44provides this output to a processor 48. The collection of digitizedoutput from the X-ray receiver 44 corresponding to a single emission ofa beam of X-rays from the X-ray emitter 42 may be referred to as aprojection image of the object P, exemplarily the patient's head. Asused herein, projection images refer to a two dimensional array ormatrix of data in which each matrix elements correlate to detected X-rayflux in a corresponding pixel. It is to be understood that, inembodiments, using storage or transmission, the data may be transmittedor stored as a one dimensional sequence. It is to be recognized, that incertain embodiments, and as briefly described above, the X-ray emitter42 and the X-ray receiver 44 are held in correspondence to one anotherand rotated about the object to be imaged P, exemplarily about arotation axis. In an embodiment, the rotation axis is aligned with thecenter of the object to be imaged P. In the embodiment depicted in FIG.2, the rotation axis is aligned with a particular anatomical feature ofinterest 50 within the patient's head P. In still further embodiments,the rotation axis may be variable, and in one example may move along acircular or other path. Other techniques or alignments for the rotationaxis may also be used as will be recognized by a person of ordinaryskill in the art.

The processor 48 receives the projection image data from the X-rayreceiver 44 and the processor executes computer readable code that isstored on a computer readable medium 52. The execution of the computerreadable code by the processor 48 causes the processor to perform dataprocessing and control functions as disclosed in further detail herein,including to execute embodiments of methods of X-ray imaging, asdisclosed in further detail herein. In a non-limiting example ofprocessing and functionality carried out by the processor 48, theprocessor 48 reconstructs a 3D image from received projection images. Areconstruction as used herein is the application of an image processingalgorithm that generates 3D data from projection images, modifiedprojection images and/or other additional data as described in greaterdetail herein. The processor 48 is further connected to a graphicaldisplay 54. The processor 48 may operate the display 54 in such a manneras to present X-ray imaging data, which in embodiments, can be 2-D or3-D X-ray images. In an exemplary embodiment, as referenced above, thedisplay 54 may also include touch sensitive controls. For example, thedisplay 54 can also operates as a user input device for a clinician ortechnician to enter control parameters or commands. While not depicted,in an additional embodiment, a separate user input device may also beused for such purposes. The processor 48 is communicatively connected tothe X-ray emitter 42 to provide operation and control signals to theX-ray emitter 42, including, but not limited to controls regarding thetiming and strength of the emitted beam 46 of X-rays.

Although the embodiment of the X-ray imaging system 40 depicted in FIG.2 depicts a single processor 48, it will be recognized that inalternative embodiments, two or more processors may be used incoordination to function together to carry out the functions andoperations as described herein. Therefore, references to the processoras found herein should be interpreted to include such multiple-processorsystems.

The X-ray imaging system 40 further includes a filter 56. The filter 56includes at least one filter leaf 58. In the exemplary embodimentdepicted in FIG. 2, the filter 56 optionally, but not necessarily,includes a plurality of filter leaves 58. Each filter leaf 58 isconstructed of an X-ray absorbent material, exemplarily lead, aluminum,copper, or tungsten. In embodiments, the at least one filter leaf 58 isadjustable in position to at least partially extend into the beam 46.The portions of the beam 46 that pass through one or more filter leaves58 will be attenuated by the material of the filter leaf 58. Thus, thefilter 56 modifies the beam 46 such that, for example the beam 46includes (1) a limited field of view beam portion 60 where the X-raysreaches the object to be imaged P unattenuated by any of the filterleaves 58 of the filter 56, and (2) at least one attenuated beam portion62 where the object to be imaged P is exposed to reduced X-rayintensity. Thus, the full field of view 64 of the beam 46 includes boththe limited field of view beam portion 60 and the at least oneattenuated beam portion 62. In the embodiment shown, the full field ofview 64 includes the limited field of view beam portion 60 and twoattenuated beam portions 62. While the embodiment shown depicts portionsof the beam 46 being attenuated in a generally horizontal dimension,other embodiments may attenuate the beam 46 in a vertical dimension, orin both dimensions.

While not depicted, in FIG. 2, exemplary embodiments may also comprise acollimator positioned between the X-ray emitter 42 and the filter 56.The collimator blocks portions of the X-rays emitted from the X-rayemitter 42 to shape the emitted X-rays into beam 46. It will berecognized that in an exemplary alternative embodiment, the filter 56may also operate as a collimator in that a sufficient number of filterleaves 58 may impinge on the beam 46 for example to block portions ofthe beam 46 entirely.

In accordance with non-limiting and merely exemplary embodiments, thediameter of the full field of view may be between 30 millimeters and 180millimeters. In certain embodiments, it is preferred that the full fieldof view is greater than a maximum diameter of the object or objectportion to be imaged P—thus, for example as projection images fromvarious angles about the object or object portion to be imaged P areobtained, such projection images capture the full size of the entireobject or object portion to be imaged P. In non-limiting embodiments,the limited field of view can have a diameter of 30 millimeters-130millimeters which can present a significantly reduced field of view tofocus in on a specific anatomical structure of interest. Similarly, inembodiments, it is preferred that the diameter of the limited field ofview be at least as large as, or slightly larger than, the maximumdiameter of the anatomical structure of interest—thus, for example theentire anatomical structure of interest is captured within the limitedfield of view projection images.

In embodiments, the processor 48 provides control signals to the filter56 to operate the filter 56—e.g., to adjust the position of one or morecollimator leaves 58 either to adjust the size of the limited field ofview beam portion 60 or to adjust an amount of attenuation of the X-raysin the attenuated portions 62 of the beam 46 m or both. In anembodiment, a plurality of filter leaves 58 are provided, each leaf 58having an known X-ray absorption characteristic. Optionally, the filtercan have one filter leaf 58 for each side of beam 46, or even just asingle filter leaf 58. The known X-ray absorption characteristic of eachfilter leaf 58 can depend upon physical characteristics of the filterleaf. Two non-limiting examples of characteristics of the filter leafinclude material or thickness. In an exemplary embodiment, the filterleaf may have one or more holes or perforations that reduce the X-rayabsorption characteristic compared to filter leaves of solidconstruction. In an exemplary embodiment, the filter leaves each havethe same X-ray absorption characteristic while in other exemplaryembodiments the leaves each have different X-ray absorptioncharacteristics.

The filter 56 can be operated to position an appropriate number offilter leaves 58 extending into the X-ray beam 46 to achieve a targetattenuation based upon the strength of the X-ray beam projected frontthe X-ray emitter 42. In embodiments, the imaging system may perform acalibration procedure to map the actual attenuation of the X-rays by theat least on filter leaf 58. Natural variation in the emitted X-ray poweror intensity, as well as natural variation, impurities, andmanufacturing tolerances in the filter leaves 58, can result in somevariance in the actual X-ray intensity in the attenuated portions.Therefore, a mapping or calibration, described in further detail belowwith respect to FIG. 3, can provide a more accurate representation ofthe X-ray intensity than default assumptions.

In clinical imaging, it is desirable to limit the X-ray dosage to whichthe patient is exposed, while using sufficient X-ray intensity toachieve a high quality medical image. One previous way in which this hadbeen achieved is that when the specific anatomical structure of interest50 is smaller than the entire object being imaged P, exemplarily thepatient's head, a conventional collimator may be used to restrict thefield of view of the X-ray beam to only the size necessary to image theanatomical structure of interest. However, this has been found by theinventor to create problems in 3-D image reconstruction in thatinsufficient information regarding the anatomical structures around theanatomical structure of interest 50 is provided, which results in lowerquality 3-D image reconstruction and a higher incidence of artifacts inthe reconstructed 3-D images. Therefore, the system and method asdisclosed herein provide for additional contextual information aroundthe specific anatomical structures) of interest 50, while limiting theX-ray dose applied to the patient. In embodiments, new image processingtechniques as disclosed herein are combined with the controlledadjustment of the filter leaves 58 in the filter 56 in order toattenuate, but not completely block, the portion of the X-ray beam 46outside of the limited field of view beam portion 60. The projectionimages produced by this attenuated X-ray dose provide the context tomake appropriate adjustments to improve 3-D image reconstruction ofprojection images from the limited field of view beam portion 60, whichhas been narrowed to specifically target the anatomical structures ofinterest.

In some cases, artifact-causing objects may still appear in theattenuated portion, but these can still be identified and the ROIcompensated to remove any residual artifact. For example,artifact-causing objects, including but not limited to dental implants,are identifiable in the projection images outside of the limited fieldof view despite the attenuated X-ray dose. The identification of theseartifact-causing objects can be used to improve image quality or removethe effects of these artifact-causing objects from the 3-D imagereconstruction of the projection images from the limited field of view.In a non-limiting example, the projection images from outside of thelimited field of view are normalized with the projection images from thelimited field of view. From this normalized image a full field of viewreconstruction can be performed. Within the full field of viewreconstruction, the object of interest (e.g. the head of the patient)can be defined even though some or all of the edges of the object ofinterest are not located within the limited field of view, but ratherare located in the attenuated portions outside of the limited field ofview. In one non-limiting example this may be performed by separatingthe reconstructed image into air and non-air (e.g. the object) portionsas these air/non-air edges are still readily identifiable within theattenuated portion. Thus, from the reconstructed full field of viewimage, embodiments are able to identify the shape and/or size of theobject and locate the position of the limited field of view relative tothe entire object of interest. This information, as well as otherinformation as described in further detail herein can be used, forexample, to improve the quality of the reconstructed volume. In stillfurther exemplary embodiments, the quality of the reconstructed volumecan be improved by calibrating the reconstruction with a knownattenuation of the object of interest.

Limited field of view reconstructions may he susceptible to anon-uniform density distribution and may also be susceptible toincreased density values caused by a high density mass located outsideof the limited field of view. In an example of the non-uniform densitydistribution, an eccentric field of view projection image acquisitionand the corresponding reconstruction from a head sized object, withuniform density, results in a reconstruction image with non-uniformdensity. The artifact in this example is the non-uniform densitydistribution. The non-uniformity is caused by a reconstruction fromprojection images in which it is indeterminate if a high density masswhich appears in the projection images is inside or outside of thelimited field of view. Similarly, when the high density mass is locatedoutside of the limited field of view, this causes erroneously increaseddensity values within the limited field of view reconstruction. If themass outside of the limited field of view is located mainly in aspecific direction from the field of view, then the density values inthe limited field of view in that direction are increased more than thedensity values in other parts of the limited field of view. If thelimited field of view is located centrally to the mass, then while thismay result in minimal non-uniformity artifacts as described above, theabsolute density values of the entire limited field of view are shiftedupward (e.g. by an unknown amount). Therefore, by locating the limitedfield of view, and the reconstruction from the limited field of view,within the object of interest, determinations can be made regarding thelocation of masses of increased density located within or outside thelimited field of view and the reconstruction of the limited field ofview thus improved as described in further detail herein.

FIG. 3 is a flow chart that depicts an exemplary embodiment of a methodof X-ray imaging 100 in accordance with the invention. It is to be notedthat the method 100 is presented merely as an example, and otherembodiments may include more or fewer steps than those depicted in themethod 100, or may perform such steps in an alternative order whileachieving the same or similar functions as disclosed herein. At 102 theposition of at least one filter leaf is adjusted in order to define oneor both of the limited field of view diameter of the unattenuatedportion of the X-ray beam and the attenuation of the X-rays in theattenuated portion of the X-ray beam. In some embodiments, the filteradjustment includes consideration of the desired X-ray intensity, whichmay be selected based upon the object to be imaged, and thus may requiremore or fewer filter leaves to achieve the appropriate attenuation.

Next at 104, X-rays are projected from an X-ray emitter in the directionof the X-ray receiver. The object to be imaged P, exemplarily the headof a patient, is disposed in the path of the projected X-rays betweenthe X-ray emitter and the X-ray receiver. The X-rays can be projected inany of a variety of beam shapes, including, but not limited to, a fan, aline, or a cone, although it will be understood that other beam shapesmay also be used. In the exemplary embodiment described herein, theX-ray beam is of a cone shape. In an embodiment wherein the X-rays areprojected in a cone-shaped beam, the dimensions of the beam and theX-ray receiver can be selected to be larger than a maximum diameter ofthe object to be imaged P, thus fully capturing the object to be imagedP within the beam. The X-ray imaging system can control a power orintensity of the X-ray beam, which can be increased or reduced dependingupon the specific object to be imaged P or the anatomical structures ofinterest within the object to be imaged P.

At 106 the projected X-rays are filtered by a filter that includes atleast one filter leaf and is disposed at a position between the X-rayemitter and the object to be imaged as positioned at 102. The at leastone filter leaf, which can be, for example, a plurality of filterleaves, partially extends into the X-ray beam and attenuates thoseportions of the beam to define (1) a limited field of view where theX-ray beam is at full intensity, and (2) at least one attenuated portionwherein the X-ray beam has been partially absorbed by the at least onefilter leaf. Thus, the collimator is operable to (1) define a limitedfield of view focused upon the anatomical structure of interest withinthe object to be imaged P, and (2) reduce the intensity of the X-raysoutside of the limited field of view. As disclosed above, in someembodiments, the X-ray beam is collimated with a collimator before beingfiltered by the filter. In still further exemplary embodiments, thefilter may also operate as a collimator if sufficient filter leavesimpinge on the X-ray beam for example to absorb portions of the X-raybeam entirely.

The X-ray beam, which now has both a limited field of view beam portionof full intensity X-rays and at least one attenuated beam portion,passes through the object to be imaged P and is received at 108 by anX-ray receiver to acquire a projection image. In general, there is arelationship between the X-ray intensity and the signal to noise ratio(“SNR”) of the acquired image, up to the saturation point of the sensor.Therefore, the anatomical structures of interest imaged with the limitedfield of view with a full intensity of X-rays will generally have ahigher SNR in the projection image, while the portions of the object tobe imaged that receive the attenuated X-rays will have a lower SNR. Itwill be understood that in embodiments as disclosed herein a pluralityof projection images are acquired by incrementally rotating the X-rayemitter and the X-ray receiver about the object to be imaged andrepeating steps 104, 106, and 108 in order to acquire projection imagestaken from a plurality of angles about the object to be imaged P.

As noted above, the portions of the object to be imaged P that receivethe full intensity X-rays will have both higher intensity measurementsand a higher SNR than those portions of the object to be imaged thatreceived attenuated X-rays. Therefore, at 110 the acquired projectionimages are normalized to adjust for differences in the image quality andimage intensity between these portions of the projection images. In oneembodiment, the normalization is performed based upon test images thatare captured at a calibration phase for the system. The test imagestaken during the calibration of the system provide information about theintensity and distribution of the X-rays after attenuation by thecollimator, and about the impact of the attenuation on the individualpixel values of the projection images. Thus, the normalization at 110can normalize the individual pixel values of the projection images basedupon the attenuation mapped during the system calibration. The result isa set of 2D projections, each containing not only the particularanatomical feature of interest 50, but as the region surrounding thefeature of interest 50, optionally (but not necessarily) including theentire object. P. Each projection can, for example, be processed by the3D reconstruction algorithm (as described below) in the same manner asif the entire projection was created with a uniform X-ray dose. Thenormalized portions of each projection representing the region outsidethe feature of interest 50 will tend to be noisier (i.e., have lowerSNR) due to the reduced dose in those regions, but their quality issufficient to reduce truncation artifacts, object-based artifacts, adother artifacts that might otherwise appear in the 3D reconstruction ofthe feature of interest 50, while the high-SNR portions of theprojections allow for a high-SNE 3D reconstruction of the feature ofinterest.

At 112 a 3-D image is reconstructed from the normalized projectionimages. In this 3-D reconstruction, the object to be imaged P isreconstructed using the projection image data acquired in both thelimited field of view and the attenuated portions of the X-ray beam. The3-D image reconstruction can be performed using a variety of techniquesand algorithms as may be recognized by a person of ordinary skill in theart.

At 114 the size of the object to be imaged P is identified. Since theanatomical structure of interest is typically located internally to theobject to be imaged P, the exterior sides or edges of the object to beimaged P will be imaged by the attenuated portions of the X-ray beam.While the attenuated X-rays limits the detail and/or quality of thereconstructions of these portions of the object to be imaged P, even atthis reduced image quality, the exterior edges of the object to beimaged P can be determined due to the inherent contrast between objectand air in the projection images. Once the object size and edges areidentified, the limited field of view around the anatomical structure ofinterest can be identified and located within the larger defined imageobject. In non-limiting embodiments, the field of view about the objectto be imaged is between 30 millimeters and 180 millimeters while thelimited field of view diameter is between 30 millimeters and 130millimeters wherein the limited field of view is smaller than the fullfield of view.

Alternatively, or in addition, one or more artifact-causing objects maybe identified in the 3-D reconstruction outside of the limited field ofview. The identification of the artifact-causing objects can be used asdisclosed herein to improve the quality of the reconstruction of thelimited field of view. As described above, these objects may be higherdensity objects that produce artifacts in the density distribution ofthe limited field of view reconstruction. In an embodiment wherein theartifact-causing object is a mass is located outside of the limitedfield of view, identification of the mass can be used to estimate theerroneous increase in density of the limited field of viewreconstruction.

Once the limited field of view is identified within the larger definedobject to be imaged P, this additional information regarding position ofthe limited field of view within the imaged object is used at 116 todefine at least one corrective parameter for use in the 3-Dreconstruction of the projection images from the limited field of view.The at least one corrective parameter can include, for example, thelocal mass distribution in the anatomical structures outside of thelimited field of view and/or positional information related to therelative positions and size of the limited field of view within theimaged object. The at least one corrective parameter can be a functionor value that is able to eliminate the non-uniform distributions ofdensity from the limited field of view reconstructed image bysupplementing the incomplete information with deduced extrapolatedand/or generalized information of the full objects. In an embodiment,the corrective parameter may be a linear function, exemplarily afunction of a 3D plane. Such corrective parameter is applied to orreplaces the density distribution to improve overall densitydistribution in the reconstructed image. Corrective parameters may inother embodiments be produced empirically, by simulation, oranalytically.

Embodiments as disclosed herein deduce information about an object'sfull size (e.g. in horizontal dimensions) when that object extendsoutside of the limited field of view. As non-limiting examples,embodiments achieve this additional information by: 1) a tworeconstruction solution whereby the object's full size is deduced from afirst reconstructed volume image, and 2) an object's full size isdeduced directly from projection images using known projection image ofposition geometry.

In the two reconstruction embodiment, before a first reconstruction, theprojection images are segmented into modified projection images of twocomponents: an air component (no attenuation) and an object component(e.g. the rest of the projection image data). Next, modified projectionimages are created from the segmented projection images. The modifiedprojection images constitute image data only from one of the aircomponents and the object component in an embodiment, the objectcomponent is further represented with a homogenous attenuation. The fullobject (e.g. head) size information is deduced from reconstructions ofthe modified projection images. In the second reconstruction, the fullobject size information is used to supplement the incomplete informationavailable from the limited field of view projection images.

In the embodiment wherein the full object's size is deduced directlyform the projection images, often an object's outer edge (e.g. theskin-air boundary) is visible somewhere near an edge of the projectionimages. From the projection images, even in the attenuated portion ofthe projection images, the skin-air-edge may be identified. Byidentifying the skin-air boundaries, this embodiment directly determinesan amount of the object that exists out of the limited field of view.

Once the object full size information is deduced, exemplarily in one ofthe two embodiments as discussed above, this information can be used toimprove the reconstruction of images, e.g. human medical and 3D dentalapplications. These reconstructions ma be improved by further assumingthat the head average density is roughly the same as with water. Thedensity of the portions of the object outside of the limited field ofview can be assumed to be the density of water or another density aspredetermined in an application or selected by a user. At least onecorrective parameter, as mentioned above, can be applied to theprojection images, exemplarily to the projection image intensities thatrepresent the object attenuation.

In an ideal condition, without the truncation of the projection image tothe limited field of view projection images, the total attenuationrepresented in the projection image is independent from the scan angle.This ideal condition is only true if the whole object is fully visiblein all of the projection images. When the limited field of viewprojection images are used, the ideal condition can be approximated byadding the missing parts of the full object at a generalized density tothe projection images by extrapolation if the full object size has beendetermined, exemplarily in one of the manners as explained above. Byextrapolating the projection images in the approximated missing parts ofthe projection images to the total attenuation of projection images ofthe full object can be normalized to thus be the same between projectionimages and independent from projection angle. In an embodiment, theextrapolated attenuation is redistributed within each projection imageby keeping the total attenuation per scan angle constant. Redistributionof the values from the portions of the object outside of the limitedfield of view can be uneven and approximate the original full object asmuch as possible. In a non-limiting embodiment, if there is onlytruncation of a left side of an imaged object then the redistributionwill be mainly shared to the left side of the limited field of viewprojection images.

Limited field of view projection images contain some detail from outsidethe limited field of view volume because each projection image is atwo-dimensional representation of X-rays passing through a threedimensional section of the object being imaged. In addition, the definedat least one corrective parameter can contain information regarding thelocation of the anatomical structure of interest within the imagedobject. This provides context for the 3-D reconstruction of the limitedfield of view projection images, which helps to identify that imageinformation from outside the limited field of view volume found withinthe limited field of view projection images. As described above, bylocating the anatomical structure of interest within the imaged object,information regarding the location of increased density masses isobtained. By determining whether these masses are within or outside ofthe limited field of view reconstruction, non-uniform densitydistribution or erroneously increased density value artifacts may beidentified and corrected. For example, by identifying a location of anincreased density mass outside of the limited field of view, the totaldensity distribution of the full object can more accurately be known andthe density distribution within the limited field of view projectionimage corrected to be reflective of this information. In a furtherembodiment, the at least one corrective parameter is based upon anartifact-causing object in the reconstruction. The corrective parametercan he used to reduce or eliminate the effects of this artifact-causingobject found in the reconstruction of the limited field of view. Forexample, as described above, an identified non-uniform densitydistribution can be corrected by flattening the density distribution inthe reconstruction with a density value or function applied across theprojection images. Additionally, if based upon the location of a massoutside of the limited field of view, the reconstruction has anerroneously increased density distribution, the density distributionwithin the limited field of view can be corrected downwards to maintainthe density distribution of the whole object.

The at least one corrective parameter is used at 118 to reconstruct a3-D image of the limited field of view portions of the projection imagesacquired using the full intensity portion of the X-ray beam resulting ina higher signal quality. As described above, a non-uniform densitydistribution can be corrected with the corrective parameter to flattenthe density distribution in the limited field of view reconstruction.The limited field of view reconstruction may also be susceptible toinaccurate determination of density values, which may also be consideredto be a form of artifact. In a non-limiting example, a doctor orclinician may be unable to determine a true density of at least onetissue from the reconstruction (e.g. whether a bone is hard or soft orto determine an amount of error between the reconstructed density andthe actual density). In an exemplary embodiment, the correctiveparameters may be used to simulate the expected density. For example, asimulation of the whole object to be imaged and density distribution ofthe whole object can yield an estimate of an expected density of aparticular anatomical object, or an expected density distribution acrossthe whole object. A density value from this simulation may be used todecrease the error in the reconstructed density value. In embodiments,the development of the at least one corrective parameter may include theuse of presumptions of object density and density distribution withinthe limited field of view. As the presumptions become more sophisticatedand detailed, the resulting corrective parameters may further improvelimited field of view reconstruction. The reconstructed 3-D image of thelimited field of view portion of the projection image can be presentedon a graphical display or stored on a computer readable medium for lateraccess by a user.

Embodiments of the system and/or method disclosed herein may be used inconnection with an X-ray imaging device that is capable of providingnormal full field of view imaging or limited field of view imaging.Exemplarily, full field of view imaging may be performed by adjustingthe at least one collimator leaf to be outside the X-ray beam forexample no portion of the X-ray beam is attenuated. Similarly, standardlimited field of view imaging may be performed by using a plurality ofcollimator leaves for example most or all of the X-rays in theattenuated portions of the X-ray beam is absorbed by the combinedcollimator leaves and thus only the limited field of view portion of theX-ray beam passes through the object to be imaged and is imaged.

It has been discovered that in the desire to limit X-ray dosage inpatients, the reduction of the field of view diameter can eliminateimportant contextual information used to provide a high quality 3-Dreconstruction. Therefore, the presently disclosed system and methodprovides a solution whereby the X-ray dose can be limited by the use ofa small diameter limited field of view projection for the 3-Dreconstruction that can be tightly adjusted to the dimensions of theanatomical structure to be imaged while attenuated X-rays areexemplarily applied to the surrounding area or peripheral area of theobject to be imaged in order to acquire the contextual information usedto achieve a high quality 3-D reconstruction of the limited field ofview projection images.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral languages of the claims.

What is claimed is:
 1. An X-ray imaging system, the system comprising:an X-ray emitter configured to produce a first X-ray beam; a filterdisposed to receive the first X-ray beam and output a second X-ray beamcomprising a full intensity beam portion and an attenuated beam portion,the filter comprising at least one filter leaf adapted to attenuate aportion of the first X-ray beam to produce the attenuated beam portion,the at least one filter leaf being adjustable to adjust at least one ofan intensity of the attenuated beam portion and a geometry of theattenuated beam portion, the X-ray emitter and filter being arranged todirect the second X-ray beam toward an object; an X-ray receiverpositioned to receive unabsorbed X-rays from the second X-ray beam,wherein the filter is disposed between the X-ray emitter and the X-rayreceiver, the X-ray receiver configured to produce a plurality ofprojection images of the object from the received unabsorbed X-rays,each of the plurality of projection images comprising a first regiongenerated from the attenuated beam portion and a second region generatedfrom the full intensity beam portion; and a processor in communicationwith the X-ray receiver, the processor adapted to execute computerreadable code stored upon a computer readable medium, and upon executionof the computer readable code, the processor receives the plurality ofprojection images from the X-ray receiver, reconstructs a first threedimensional image from the plurality of projection images from the X-rayreceiver, and determines at least one corrective parameter from thefirst three dimensional image.
 2. The X-ray imaging system of claim 1,wherein the processor reconstructs the first three dimensional image bynormalizing at least one of the first and second regions of eachprojection image to produce a set of normalized projection images, andreconstructing the first three dimensional image based upon thenormalized projection images.
 3. The X-ray imaging system of claim 1,wherein the processor further produces a second three dimensional imagebased at least upon the at least one corrective parameter and the higherdose regions of the projection images.
 4. The X-ray imaging system ofclaim 1, wherein the processor locates a limited field of view withinthe first reconstructed three dimensional image.
 5. The X-ray imagingsystem of claim 4, wherein the processor produces a second threedimensional image of the limited field of view within the reconstructedthree dimensional image using the at least one corrective parameter. 6.The X-ray imaging system of claim 5, wherein the processor locates thelimited field of view in the first reconstructed three dimensionalimage.
 7. The X-ray imaging system of claim 4, wherein the at least onecorrective parameter is a function to smooth non-uniform distributionsof density from the limited field of view.
 8. The X-ray imaging systemof claim 4, wherein the at least one corrective parameter is an estimateof the expected density distribution across the object of interest. 9.The X-ray imaging system of claim 1, further comprising a graphicaldisplay operable by the processor to present the reconstructed threedimensional image based upon the selected portion of the object.
 10. Amethod of X-ray imaging, comprising: projecting a first X-ray beam froman X-ray emitter; filtering the first X-ray beam with a filter toproduce a second X-ray beam comprising a full intensity beam portion andan attenuated beam portion, the filter comprising at least one filterleaf that absorbs at least a portion of the projected X-rays to producethe attenuated beam portion; receiving X-rays from the second X-ray beamat an X-ray receiver to acquire a plurality of projection images, eachof the plurality of projection images comprising a first regiongenerated from the attenuated beam portion and a second region generatedfrom the full intensity beam portion; reconstructing, with a processor,a first three dimensional image from the plurality of projection imagesfrom the X-ray receiver; and determining at least one correctiveparameter based upon the first three dimensional image.
 11. The methodof claim 10, further comprising: normalizing at least one of the firstand second regions of each projection image to produce a set ofnormalized projection images; and reconstructing the first threedimensional image based upon the normalized projection images.
 12. Themethod of claim 10, further comprising producing a second threedimensional image based at least upon the at least one correctiveparameter and the higher dose regions of the projection images.
 13. Themethod of claim 10, further comprising locating, with the processor, alimited field of view within the first reconstructed three dimensionalimage.
 14. The method of claim 13, further comprising producing a secondthree dimensional image of the limited field of view within thereconstructed three dimensional image using the at least one correctiveparameter.
 15. The method of claim 10, wherein the at least onecorrective parameter is a function to smooth non-uniform distributionsof density from the limited field of view.
 16. The method of claim 10,wherein the at least one corrective parameter is an estimate of theexpected density distribution across the object of interest.
 17. Themethod of claim 10, further comprising presenting the firstreconstructed three dimensional image on a graphical display.
 18. Themethod of claim 10, further comprising locating a limited field of viewwithin the plurality of projection images by identifying a size of anobject to be imaged and a position of the limited field of view withinthe plurality of projection images.
 19. The method of claim 10, whereinthe at least one corrective parameter is based upon the identified sizeof the imaged object and the identified position of the limited field ofview in the reconstructed first three dimensional image.
 20. The methodof claim 10, wherein the at least one filter leaf comprises a pluralityof filter leaves, the method further comprising adjusting relativepositions of the plurality of filter leaves to control the attenuationof the X-rays in the attenuated beam portion.